The Research of the Synthesis Mechanism and Synthesis Process of High Crystallinity Globular

In figure 1, at 500 0C, there were diffraction peaks of Na2B4O7, CO(NH2)2, B2O3, Na2O, carbon (C) and metaboric acid (HBO2) phases on the XRD diffraction pattern which meant raw material of Na2B4O7·10H2O made reaction of dehydration and decomposition, decomposed to Na2O and B2O3. When the synthesis temperature rose to 600 0C, there were not only diffraction peaks of Na2B4O7, CO(NH2)2, B2O3, Na2O, C and HBO2 phases on the XRD, but also was diffraction peak of h-BN. It meant that h-BN began formed, part of B2O3 reacted nitriding reaction in the flowing NH3 atmosphere to form h-BN. Meanwhile undecomposed Na2B4O7 reacted with urea (CO (NH2)2) or flowing NH3 to generate h-BN. With the synthesis temperature rising to 700 0C, the diffraction peaks of Na2B4O7 and CO(NH2)2 phases disappeared which suggested reactions came to an end and h-BN mainly formed. In other words, synthesis process of h-BN was a nitriding process of new formed B2O3 with high activity in flowing NH3 atmosphere. In addition, diffraction peaks of the HBO2 and Na2O phases disappeared and the diffraction peak of sodium metaborate (NaBO2) emerged on the XRD, indicating that HBO2 and Na2O reacted to generate a new phase of NaBO2. Uniform distributive NaBO2 played a supporting role, which was conducive to the flow of NH3 atmosphere and promoted the synthesis of h-BN. But on the other hand it would lose part of B to decline the production h-BN, so borax content cannot be too much.

Figure 2 was the XRD diffraction pattern of the samples for different synthesis temperatures. It could be seen from the figure that diffraction peaks of typical (002) and (100) crystal planes of h-BN appeared on the XRD diffraction pattern, which meant the obtained samples were h-BN at 700-900 0C. With the increase of the synthesis temperature, the diffraction peak intensity of (101), (102), (104), (110) and (112) crystal planes of h-BN increased gradually. For example the half high width of (002) diffraction peak fell from 0.71o at 700 0C to 0.35o at 900 0C, which showed that with the increase of the synthesis temperature, three-dimensional orderly degree of h-BN increased, the crystallinity increase gradually.

Figure 2. The XRD diffraction pattern of synthesized Figure 3. The XRD spectrum of synthesized

powders at different temperatures samples at different temperatures for 3h